"We think we've made progress during some rounds, but as we said coming out of the last one, we hadn't seen enough made. We hadn't seen enough realism, quite frankly, on the table," said deputy State Department spokeswoman Marie Harf.

The talks, which by late Monday had already lasted for five hours in Geneva, were aimed at working out ahead of time potential sticking points once the negotiations with world powers, known as the P5+1, resume next week in Vienna.

"It is a consultation round to talk about the wide range of issues and exchange views leading up to the next negotiating round in Vienna and, of course, feeding into that," Harf told reporters.

US and Iranian negotiators began two days of direct talks earlier in Geneva to discuss what Harf said were "a range of topics" around the nuclear negotiations involving the five permanent Security Council members plus Germany.

Those negotiations are facing a July 20 deadline for turning a temporary deal struck over Iran's nuclear program into a permanent agreement.

"We know we don't have a lot of time left. That's why we've said diplomacy will intensify," Harf said.

"People need to make tough choices, but we are very focused on that July 20th time."

Washington and its allies are seeking solid commitments that will ensure Iran's stated desire for a peaceful atomic power program is not a covert attempt to build a nuclear bomb.